1. Technical Field
The present invention relates to microelectronic packaging structures and particularly to packaging structures that include bonding the semiconductor device to a chip carrier or heat spreader employing a reworkable and thermally conductive adhesive. In addition, the present invention relates to novel reworkable and thermally conductive adhesives.
2. Background of the Invention
A very common method of interconnecting a semiconductor die to other devices uses an adhesive, referred to as a die attach adhesive, to bond the passive or back side of the die to a substrate containing interconnect wiring (referred to as a chip carrier). The die is also electrically connected to the chip carrier with wirebonds from peripheral 1/O pads on the die to corresponding pads on the carrier. The die attach adhesive must be cured prior to the wirebonding operation to make certain that the chip does not move during wirebonding. With the usual die attach adhesives, rework of a misaligned or defective chip, or one on which a wirebonding defect has been found by inspection, is then impossible because the adhesive is an insoluble thermoset plastic. This is a significant limitation because partially good assemblies cannot be reworked to recover the value of the good components of the assembly.
Although thermoplastic die attach adhesives have been used for reworkability, their adhesive characteristics are inferior to the more preferred thermoset materials used for this purpose. Moreover, the thermoplastic materials exhibit certain process disadvantages over a thermoset. In one case, the thermoplastic is a paste, which is dissolved in a solvent. If the die is placed before solvent evaporation, there will be a lot of voids in the bond line after bonding. If the solvent is evaporated before die placement, then there is a firm thermoplastic bump/solid that will require chip placement under heat, pressure for some time to accomplish bonding.
It is difficult to achieve thin bond lines, typical of die attach processing ( less than 0.002xe2x80x3) with a dried thermoplastic bump. The thermoplastic resists flow because of its inherently much larger molecular weight (compared to an unreacted thermoset monomer) and in combination with very high filler loadings become even more difficult to spread.
An alternative thermoplastic approach is to provide precast films of thermoplastic. No solvent dry process is needed and the films are already in a thin bond line thickness (e.g. 0.001, 0.002, 0.003xe2x80x3). However, these still have a big process downside. Bonding flat-to-flat surfaces with films will always result in air entrapment (at least 30% at one interface and more typically  greater than 50% at both interfaces). Air entrapment can be eliminated with a vacuum lamination or vacuum bagging with autoclavexe2x80x94two non standard, and expensive batch processes. Film bonding also requires temperature, pressure and process time.
Accordingly, there exists a need for providing improved adhesives for such purposes.
The present invention provides an adhesive that is reworkable along with making it possible to fabricate packaging structures providing functionality and reliability equivalent to conventional wirebonded or flipchip packaging. For multichip (MCM) package configurations, reworkability is of particular value because it avoids the loss of valuable chips in partially good MCMs.
In particular, the present invention relates to a packaging structure comprising at least one semiconductor device bonded to a chip carrier or heat spreader with an adhesive wherein the adhesive is reworkable and thermally conductive and comprises a cured reaction product from a diepoxide wherein the epoxy groups are connected through an acyclic acetal moiety; and a thermally conductive filler.
Another aspect of the present invention relates to a method for fabricating packaging structure which comprises bonding at least one semiconductor device to a chip carrier or heat spreader by applying a thermally conductive adhesive composition comprising a diepoxide wherein the epoxy groups are connected through an acyclic acetal moiety; and; curing the composition to provide a reworkable and thermally conductive adhesive.
A still further aspect of the present invention is conceived with reworkable, thermally conductive adhesive composition comprising A) about 28 to about 60% with a cured reaction product from a diepoxide wherein the epoxy groups are connected through an acyclic acetal moiety, B) about 38 to about 70% by weight of a thermally conductive filler; and C) about 0.09 to about 2% by weight of a thixotropic agent based upon the total A, B, and C in the composition.
The present invention provides advantages of thermosets and the rework advantages of thermoplastics. The present invention makes possible structures wherein the bond line is void- free or free from air entrapment.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.